Optical biosensors based on surface plasmon resonance (SPR) are today widely used for analyzing a wide range of biological and chemical interactions. SPR biosensors allow the determination of the affinity and kinetics of molecular interactions in real time without the need for a molecular tag or label. Analytes, or ligands, in a solution are contacted with a sensing surface with immobilized binding partner, or receptor. Binding of analyte to surface-bound binding partner alters the refractive index at the sensing surface, and this refractive index change can be monitored to measure accurately the amount of bound analyte, its affinity for the receptor and the association and dissociation kinetics of the interaction. Commercial SPR biosensor systems are available which permit a high degree of automation and parallelization, and may therefore be used for high throughput screening assays.
However, a negative feature of SPR biosensors is their sensitivity to the optical properties of the samples. While such bulk effects as well as effects of signal drift and non-specific binding are usually corrected for by the use of a reference surface, the sample bulk effect may in some instances be so large that they can mask the specific binding signal. This is, for example, the case when studying interactions of small molecules, i.e. of low molecular weight, such as in so-called drug libraries, which as a result of their limited solubility in aqueous solutions usually have to be dissolved in buffers containing organic solvents, typically dimethyl sulfoxide (DMSO). DMSO has a high refractive index, and small differences in concentration will therefore induce large increases in response signal that have to be subtracted.
Today, this is variance is corrected for by performing a rather sophisticated and laborious calibration procedure, often referred to as solvent correction, involving the creation of a calibration curve by running negative controls with varying DMSO contents.
It is an object of the present invention to provide a method for screening compounds in drug libraries and the like which does not require a separate solvent correction step involving separate runs with calibration solution.